In order to perform electronic control of the variable valve timing control, the variable valve timing controller which has the motor as a source of the drive has been developed. The variable valve timing controller described in JP-2006-70754A (US2006/0042578A1) includes a first gear, a second gear, a phase changing gear, and an electric motor. The first gear (outer gear) is concentrically arranged with the camshaft and is rotated with the rotation driving force of the crankshaft. The second gear (inner gear) rotates together with the camshaft. The phase changing gear (planet gear) transmits the torque of the first gear to the second gear, and varies the rotational phase of the second gear relative to the first gear. The motor is coaxially provided to the camshaft so that the revolution speed of the phase changing gear is controlled. The number of teeth of the first gear, the second gear, and the phase changing gear is determined so that the camshaft may rotate with one half of the rotational speed of the rotational speed of the crankshaft. And when not varying valve timing, the rotational speed of the motor coincides with the rotational speed of the camshaft, and the revolution speed of the phase changing gear coincides with the rotational speed of the camshaft. When varying valve timing, the motor speed is varied relative to the rotational speed of the camshaft, and the revolution speed of the phase changing gear is varied relative to the rotational speed of the camshaft. Thereby, the difference between the rotational phase of the first gear and the second gear is varied.
In the above motor drive variable valve timing controller, as a driving current of the motor (“motor current”) increases during the variable valve timing control, the heat value of the motor increases and a coil temperature rises. The motor current is feedback controlled so that a deviation between the target motor speed and the actual motor speed is decreased. If it is continued that the deviation is larger than a predetermined value for a long period, there is a possibility that the motor current increases and the coil temperature of the motor may exceed allowable temperature, which causes durability deterioration and malfunction of the motor.